1. Field of the Invention
This invention relates to aqueous metal oxide slurries and particularly to aqueous titanium dioxide slurries.
2. Brief Description of the Prior Art
Stable high solids concentration aqueous titanium dioxide slurries suitable for direct incorporation into aqueous paint or paper media represent the possibility of large savings in costs to the paint manufacture in comparison with the use of dry titanium dioxide powder. These slurries must have a solids content greater than 60% and a low viscosity which does not change substantially on storage. Such slurries have been produced by pigment manufacturers from dry titanium dioxide powder with the aid of a wide variety of dispersing agents and viscosity stabilisers and give sufficient dispersion stability for use after transport and storage.
The production of these slurries has transferred the step of dispersion from the paint manufacturer to the pigment manufacturer so that the cost of manufacturing the pigment is necessarily increased.
Titanium dioxide, produced either by the vapour phase oxidation of titanium tetrachloride, or by the hydrolysis of aqueous solutions containing titanium sulphate to produce hydrous titanium dioxide followed by calcination hereafter referred to as the "sulphate process", is generally treated by forming a dispersion of the titanium dioxide in water and precipitating hydrous metal oxides in the dispersion to form a coating thereof on the titanium dioxide which is then recovered and dried to produce a final pigmentary product. A considerable reduction of energy consumption could be achieved if it were possible to produce a high solids concentration titanium dioxide slurry or a proportion thereof directly from the wet treated pigment, omitting or minimising the drying step, and it would also be a considerable advance in the art if the use of dispersing agents could be avoided or minimised.
Aqueous paint media are usually alkaline and negatively charged and it is for this reason that the titanium dioxide slurry, to be compatible with such media, requires a negative charge. In practice the slurry should have as high a negative charge and as high a pH as practicable, preferably a pH of at least 9.2 although at above a pH of about 10.5 any alumina present on the titanium dioxide particles may dissolve. The negative charge of the slurry at a given pH is related to the isoelectric point of the slurry, that is, the pH at which the charge is zero, and the lower the isoelectric point the higher the negative charge at a given alkaline pH. The isoelectric point is a property of the surface of the particles of the titanium dioxide in the slurry and may be controlled by coating the surface with metal oxides. For example, a coating of alumina gives an isoelectric point at a pH of 9.2 and a coating of titania gives an isoelectric point at a pH of 4.2. A homogeneous mixed alumina/titania coating gives an intermediate isoelectric point.
The composition of the metal oxide coating also affects the physical nature of a slurry of the coated titanium dioxide in other ways and, for example, a dispersion of titanium dioxide having a coating substantially consisting of hydrous titania is very difficult to filter and wash. The ease of filtering improves if a larger proportion of hydrous alumina is incorporated in the coating but at higher proportions of alumina the isoelectric point increases so that the pigment is less negatively charged at a given pH and is less suitable for incorporation in aqueous paint media. The manufacture of titanium dioxide slurries suitable for incorporation in aqueous paint media by processes involving coating the titanium dioxide with metal oxides is, therefore, subject to a combination of problems and an attempt to alleviate one only serves to exacerbate the others.
We have found that the properties of a metal oxide coated titanium dioxide can be altered after precipitation by ageing at an elevated temperature so as to produce in at least some circumstances, titanium dioxide which is capable of forming a slurry more suitable for incorporation in aqueous paint media than might have been expected. Without being so limited we believe that this may be due to an alteration in the isoelectric point of the slurry as a result of the effect of the hot ageing step on the surface properties of the coated titanium dioxide particles.